1. Field
The present invention relates to microelectromechanical devices and especially to an improved pressure sensor structure and a pressure sensor according to preambles of the independent claims.
2. Description of the Related Art
Pressure is a physical quantity that corresponds to the ratio of force acting on a surface to the area of the surface. A device that can be used as a gauge to measure the pressure is a pressure sensor.
Atmospheric pressure is the pressure exerted at the surface by a column of air in an atmosphere. The atmospheric pressure varies with altitude and weather patterns. The quantity obtained with a pressure sensor may exclude the ambient atmospheric pressure and in this case indicates overpressure. If atmospheric pressure is included, the result indicates absolute pressure.
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have some sort of mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronic elements can be fabricated on a piece of silicon to enable various types of devices.
FIG. 1 illustrates an exemplary structure of a microelectromechanical device for sensing of pressure. Microelectromechanical pressure sensors typically comprise a thin diaphragm 10 that is spanned over a gap 12 that provides a volume at a reference pressure. The diaphragm deforms due to difference between the reference pressure and an ambient pressure surrounding the sensor. The diaphragm displacement may be translated to an electrical signal with capacitive or piezoresistive sensing.
With microelectromechanical devices, the dimensions are very small; the components typically range in size from tens of micrometers to some millimeters. This imposes many challenges to the design. For example, in a microelectromechanical pressure sensor, the detected diaphragm displacement due to pressure change can be nanometers or less. This means that signals generated by the displacements are small. Already change of dimensions of the elements caused by thermal expansion in varying operating temperatures can cause significant variations to the measured values. In addition, the sensor structure and the associated electronics need to be capsulated in a package. The package may have different thermal expansion coefficient than the sensor structure, which may cause large, temperature dependent bending stresses that distort the measurements.